This invention relates to a low alloy cast steel which affords extremely high wear- and impact-resisting properties and also relates to a process for producing the low alloy cast steel.
Cast steel parts as used in machines and equipments in the mining, civil engineering and cement industries should afford excellent wear- and impact-resisting properties.
Hitherto, wear resisting cast steel parts have been made of high-chromium cast iron, high-manganese cast steel, high chromium cast steel, low alloy cast steel and the like. However, there is not known a cast steel which provides excellent toughness and wear-resisting property at the same time, so that the aforesaid cast steels must have selectively been used according to requirement.
Low alloy cast steels, however, provide relatively well balanced toughness and wear resisting property. High manganese cast steel may be comparable to the low alloy cast steel, because of its toughness 10 times as high as that of a low alloy cast steel, although the wear resistance thereof remains only two thirds of the low alloy cast steel. Thus, it has, for long time, been a demand to provide a low alloy cast steel which has both high impact resisting property as is afforded by a high manganese cast steel, and wear-resistance twice that of a conventional low alloy cast steel, whereby a considerably wide range of application of high manganese cast steels may be substituted by that of such low alloy cast steels.
There is a relationship between impact values and hardness of a conventional low alloy cast steels which is well known to the skilled in the art. As can be known from the prior technology of the field, the low alloy cast steels having impact values of a range from 1.8 to 2.0 kg-m/cm.sup.2 insure the freedom of a failure and thus are reliable in use. However, these are short of desired hardness and wear-resisting property. Hardness of Rockwell C 60 leads to improvements in wear-resisting property, but cause a failure sometimes. In short, low alloy cast steels having over hardness of Rockwell C 50 and an impact value of 4 kg-m/cm.sup.2 provide desired wear-resisting property and toughness.
Meanwhile, our investigations concerning a cast steel have posed problem to be solved. More particularly, when the raw materials of the cast steel are melt in a low frequency induction furnace or the like, in case that a melting temperature is too high or the time held for melting is excessively long, such as a melting temperature over 1650.degree. C. and a melting time of more than one hour, nitrogen gas is dissolved from the air into a melt in a great quantity, thereby producing aluminum nitride in the steel due to the reaction of nitrogen with aluminum contained in the steel. Aluminum nitride precipitates along primary cristal grain boundaries of a steel, upon solidification, thereby providing the so-called rock-candy fractures.